Modifying the Xiang Sheng 708B [56K!!!!]

[Miller-8]

Guys I have a question for some of you. You are spending quite a bit on things like caps and resistors but all of you seem to have fitted an Alps Blue pot. Whilst it's a decent pot it is by no means the best. Why has nobody tried a Noble or a TKD or an Alps Black? The volume pot is one of the major components that affects sound quality of a pre-amp after all.

[fishcarver]

Excellent tutorial! Very well written and with all the photos it is first rate. Thank you for posting this thread.

[dcheming]

Originally Posted by Miller-8

Guys I have a question for some of you. You are spending quite a bit on things like caps and resistors but all of you seem to have fitted an Alps Blue pot. Whilst it's a decent pot it is by no means the best. Why has nobody tried a Noble or a TKD or an Alps Black? The volume pot is one of the major components that affects sound quality of a pre-amp after all.

I got the Blue Velvet way back when I first started modding this amp and I wasn't really sure how much money I wanted to put into it at the time. Considering how much the ALPS helped I think it would definitely be worth it to get an even better one and I would love to try a TKD when I have the money.

[Chops]

Originally Posted by dcheming

I got the Blue Velvet way back when I first started modding this amp and I wasn't really sure how much money I wanted to put into it at the time. Considering how much the ALPS helped I think it would definitely be worth it to get an even better one and I would love to try a TKD when I have the money.

x2

I too wanted to wait and see how much the amp improved with the mods I did before deciding to get a TKD or Black Beauty, which BTW I was originally planning on getting before deciding on the Blue Velvet.

However, with the way things have turned out so far with this amp, I'm now very seriously considering the TKD for an upgrade. This amp really IS that good.

[daggerlee]

quite and outstanding writeup guys!!

[dcheming]

Well apparently I had looked at the pinout diagram of the 6X4 wrong. I thought this mod would only require two wire jumpers, but one of the two pins that needs to be swapped has to actually be moved to a new spot in the socket itself. Thankfully this extra step is very easy due to the design of the socket.


6X4 Rectifier Modification


Rectifier Pinouts

The difference between the 6Z4 and the 6X4 is that one of anodes and the cathode are connected to different pins. The 6X4 draws 600mA for it's heater and I assume the 6Z4 draws 300mA, but I have no idea since I can't find a data sheet for it.





Stock Layout

Here is the stock layout using the 6Z4 for reference.





Step 1

The first thing you'll have to do is de-solder pin 5 from the PCB. Try to clean the majority of the solder off it too.





Step 2

Bend the pin straight up and remove if through the other side of the socket.





Step 3

Next de-solder pin 7 from the PCB and bend it a bit to the left like shown in the pic.





Step 4

This step is just a safety precaution against possible shorts. Scrape away the corner of the trace near pin 7 as shown in the pic using a heavy Xacto knife or a small file.





Step 5

Craft a little wire jumper similar to the one in the pic. This will be used in step 6.





Step 6

The jumper will be connecting pin 7 to the trace that pin 5 did originally.





Step 7

This is where we reinsert the pin that we took out in step 2. Put it in the hole for pin 6 and bend the tab towards the center of the socket as shown in the pic. To avoid excess play make sure it is fully seated before you bend the tab.





Step 8

Craft a little wire jumper similar to the one in the pic. This will be used in step 9.





Step 9

For the final step, run the little jumper from pin six over to the trace that pin 7 originally connected to as shown.





Comments

This mod is fairly easy to do if you've worked with jumpers before. It's a little tedious to get them to fit right, but just take your time crafting them. If you plan on doing this mod keep in mind that you'll have to disconnect the heater in the 6N1 up front or you will burn up the transformer. The heater winding feeding the 6Z4 and the 6N1 is only rated for 600mA and since the 6X4 draws 600mA by itself, there's no way you can have the 6N1 connected too. You could probably leave the orange LEDs in the front window connected since they draw so little power.

Please be careful about keeping your work clean since these pins carry HV. Needless to say, I assume no responsibility or liability for damage to you, your amp, or your other equipment by doing this modification. If you are not confident in your soldering ability PLEASE don't attempt this mod.



Results

So far everything is running smoothly. I've been listening to the amp for about two hours and the transformer isn't overly warm, but I'll keep a close eye on its temp.

At this early stage, I would say that the amp does indeed sound better with the 6X4. The sound seems a bit more crisp and overall the sound seems a bit more detailed, but I want to give it some more time before I judge it.


Stock 6Z4 vs. Tung-Sol JTL-6X4



PS with 6X4

[Chops]

Well... Looks like I'll be planning to do another mod in the near future.

BTW, nice macro shots!

[dcheming]

Well I ran the amp for 7 hours last night and the transformer stayed at the same operating temp. So far so good.

I measured the heater voltage going to the 6X4 and it was about 7.12VAC. This isn't quite as high as the 7.42VAC that was feeding the audio tubes, but I still want to fix it also. Getting the heaters in the three audio tubes and in the new rectifier to the proper 6.3VAC is the next mod on my list.

[Chops]

Since we'll both be lowering the voltage on these heaters and NOT the grids, then the sound should pretty much remain the same. Am I correct?

Also, do you have anymore thoughts on the sound of the new 6X4?

I just remembered something kinda important... I think I might need a 6X4 for myself before I do this socket rewire mod that I was planning on doing this weekend.

[dcheming]

"Since we'll both be lowering the voltage on these heaters and NOT the grids, then the sound should pretty much remain the same. Am I correct?"

I'm not exactly sure how the sound is affected by going from too high of a heater voltage to normal. I didn't really notice anything dropping down to the 6.63VAC that I'm using now for the audio tubes, but you'll just have to critically listen to yours when you do it to see.

Speaking of dropping voltage, the 6X4 seems to drop more voltage than the 6Z4 did since the B+ now measures around 195VDC instead of around 220VDC. Since the HV was too high due to the 110V transformer anyways, this will probably be for the best. I really need to find out what B+ this amp is supposed to have and go from there.


"I just remembered something kinda important... I think I might need a 6X4 for myself before I do this socket rewire mod that I was planning on doing this weekend."

LOL! I was actually thinking last night how easy it would be to do this mod and forget this fact. That would be a bummer for sure since your amp would down until you got a 6X4.